1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus, and particularly to a fixing device having improved accuracy of temperature control in a nip portion as well as a control device thereof and an image forming apparatus.
2. Description of the Related Art
In an image forming apparatus such as a laser printer employing a dry electrophotograph system, a toner image is formed on a record paper sheet, and is fixed thereto by a fixing device.
The fixing device generally includes a fixing roller for heating and fixing the toner image onto the record paper sheet as well as a pressing roller for pressing the sheet against the fixing roller. The record paper sheet bearing the toner image passes through a portion between the fixing and pressing rollers so that the toner image is fixed to the record paper sheet.
Various techniques have been disclosed in connection with the temperature control of the fixing and pressing rollers in the above type of fixing device.
For example, a document 1 (Japanese Laid-Open Patent Publication No. 07-295436 has disclosed a technique relating to a fixing device in which a fixing roller and a pressing roller each internally having a heat source are arranged and are vertically spaced from each other, and particularly relating to temperature control of these rollers. Specifically, when heating of both the rollers is controlled in substantially the same manner, such a situation may occur that a hot airflow or the like from the lower side to the upper side heats the roller in the lower position more strongly than the roller in the lower position. For avoiding this situation, the above document has disclosed a technique for setting a ratio between the electric powers supplied to the rollers, respectively.
The fixing device generally has a portion where the fixing and pressing rollers nip the sheet to fix the toner image onto the sheet, and this portion is generally referred to as a “nip portion”. When the accuracy of the temperature control in the nip portion of the fixing device is improved, this can stabilize fixing accuracy of the toner image, and therefore can stabilize image forming performance. When ripple occurs in temperature of the nip portion, the fixing accuracy of the toner image lowers so that the image forming performance becomes unstable. Therefore, it is important to improve the accuracy of the temperature control of the nip portion.
In connection with the above, a document 2 (Japanese Laid-Open Patent Publication No. 07-334023) has disclosed a related technique. In a fixing device of the document 2, a record paper sheet passes through a portion between a heating roller and a pressing roller so that a toner image is fixed onto the record paper sheet. A heating member contained in the heating roller is supplied with an electric power proportional to a difference between its temperature and a set temperature. The temperature of the heating roller is estimated based on a quantity of electric power supplied to the heating roller in proportional control. The set temperature of the heating member changes based on the estimated temperature of the pressing roller.
In the technique disclosed in the above document 2, the heating member directly heats the heating roller in contact with the record paper sheet. The temperature of the pressing roller lowers due to nipping of the paper sheet between the heating and pressing rollers as well as other reasons, and this temperature is estimated based on the quantity of the electric power supplied to the heating member that is a target of the proportional control.
In the fixing device, it is always necessary to improve the accuracy of the temperature control of the nip portion from the viewpoint of stabilizing the image formation performance, as described above.